East Okoboji Lake IA 06-LSR-1652

Dickinson County S29T99NR36W at E edge of Okoboji

Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2014 - 2016
Stream/River Water Quality Data	CY 2014 - 2016
Lake Water Quality Data	CY 2012 - 2016
Biological Sampling Data	CY 2012 - 2016*
Fish Kill Data	CY 2012 - 2016
Fish Tissue Sampling Data	CY 2012 - 2016
Continuous Dissolved Oxygen Sampling Data	CY 2012 - 2016
*Biological sites sampled in both 2011 and 2015 were also assessed.

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Cycle: 2018
Release Status: Final
Overall IR: 2 - Some of the designated uses are met but there is insufficient data to determine if remaining designated uses are met.
Trophic: Eutrophic
Trend: Stable
Created: 3/14/2019 2:30:50 PM
Updated: 6/7/2019 12:49:52 PM

Use Support

Class A1

Recreation - Primary contact

Fully Supported

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Exotic/Invasive Species: Exotic Species

Support Level

Further Investigation Needed

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Slight

Status

Continuing

Source

Other: Exotic Species

Source Confidence

High

Cycle Added

2016

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Biological monitoring: Iowa DNR Fisheries

Class HH

Human Health -

Fully Supported

General Use

General Use water -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.” The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to an invasive species introduction (Zebra Mussels). Fish consumption uses are considered “Fully assessed.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) results of U.S. IDNR fish contaminant monitoring in 2015.

Assessment Explanation

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of East Okoboji Lake are assessed (monitored) as "fully supported" based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 61, and 66 respectively for East Okoboji Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place East Okoboji Lake in the Eutrophic category. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was relatively low at East Okoboji Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in East Okoboji Lake (2.85 mg/L) was ranked 62nd among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at East Okoboji Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (41.3 mg/L) was ranked 113th of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2012-2016. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the DNR Fisheries Bureau, this lake was infested with Zebra mussels in 2012 and the population exploded in 2015.This invasive species will likely cause water quality to degrade in the future. This potential impairment is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation.Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and no violations of the criterion for pH in 15 samples(0%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of East Okoboji Lake

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. DNR fish contaminant monitoring at East Okoboji Lakein 2015. The composite samples of fillets from channel catfish and walleye had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.05 ppm; total PCBs: <0.06 ppm; and technical chlordane: <0.02 ppm. Levels of primary contaminants in the composite sample of walleye were as follows: mercury: 0.052 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2015 sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.

Monitoring and Methods

Assessment Key Dates

5/29/2012

Fixed Monitoring Start Date

8/28/2016

Fixed Monitoring Start Date

8/10/2004

Fish Tissue Monitoring

9/11/2015

Fish Tissue Monitoring

9/18/2015

Fish Tissue Monitoring

Methods

120

Surveys of fish and game biologists/other professionals

222

Non-fixed-station monitoring (conventional during key seasons and flows)

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)